[InstCombine] Fix APInt ctor assertion

The (extended) bit width might not fit into the (non-extended)
type, resulting in an incorrect truncation of the compared value.

Fix this by using m_SpecificInt(), which is both simpler and
handles this correctly.

Fixes the assertion failure reported in:
#114539 (comment)

Author: nikic

llvm/lib/Transforms/InstCombine/InstCombineAddSub.cpp

@@ -1357,14 +1357,10 @@ Instruction *InstCombinerImpl::
   //   low bits to skip = shift bitwidth - high bits to extract
   // The shift amount itself may be extended, and we need to look past zero-ext
   // when matching NBits, that will matter for matching later.
-  Constant *C;
   Value *NBits;
-  if (!match(
-          LowBitsToSkip,
-          m_ZExtOrSelf(m_Sub(m_Constant(C), m_ZExtOrSelf(m_Value(NBits))))) ||
-      !match(C, m_SpecificInt_ICMP(ICmpInst::Predicate::ICMP_EQ,
-                                   APInt(C->getType()->getScalarSizeInBits(),
-                                         X->getType()->getScalarSizeInBits()))))
+  if (!match(LowBitsToSkip,
+             m_ZExtOrSelf(m_Sub(m_SpecificInt(XTy->getScalarSizeInBits()),
+                                m_ZExtOrSelf(m_Value(NBits))))))
     return nullptr;
 
   // Sign-extending value can be zero-extended if we `sub`tract it,

llvm/test/Transforms/InstCombine/conditional-variable-length-signext-after-high-bit-extract.ll

@@ -1137,3 +1137,18 @@ define i32 @n290_or_with_wrong_magic(i32 %data, i32 %nbits) {
   %signextended = or i32 %high_bits_extracted, %magic
   ret i32 %signextended
 }
+
+define i32 @bitwidth_does_not_fit(i3 %arg) {
+; CHECK-LABEL: @bitwidth_does_not_fit(
+; CHECK-NEXT:    [[NEG:%.*]] = sub i3 0, [[ARG:%.*]]
+; CHECK-NEXT:    [[NEG_EXT:%.*]] = zext i3 [[NEG]] to i32
+; CHECK-NEXT:    [[SHR:%.*]] = lshr i32 1, [[NEG_EXT]]
+; CHECK-NEXT:    [[INC:%.*]] = add nuw nsw i32 [[SHR]], 1
+; CHECK-NEXT:    ret i32 [[INC]]
+;
+  %neg = sub i3 0, %arg
+  %neg.ext = zext i3 %neg to i32
+  %shr = lshr i32 1, %neg.ext
+  %inc = add i32 %shr, 1
+  ret i32 %inc
+}